It all depends on where you are. In principle, a high quality restaurant is a good business - but not if it's going to be sited in a run down area with high unemployment, surrounded by cheap, fast food outlets! What I am trying to say is, whilst you may have a good idea for a business, that doesn't necessarily mean it's going to be successful.
You need to do market research - find out if people in your area want and would use a service like this. The key is in how you word your questions - you want honest answers, you don't want the people you question to give you the answer they think you want to hear. So, you don't ask them if they would like the service you are proposing straight off, you find out what they presently do to get these items, are they happy doing it that way, if there was anything that could make it easier for them, what is their prime consideration when shopping for these things - convenience, price, range, etc.
You need to take a look at what competition there is - and by that I don't just mean places offering the exact same service, but those places that are alternatives, for example, the cake shop, the card shop, the stationery shop, the toy shop, the fancy dress shop etc etc, even the supermarkets because most of them sell greetings cards, cakes, flowers, balloons etc, as well as all the other party food and goodies.
This information will give you a better idea of how likely your business idea is to succeed in your area.
Hope it helps.